FIG. 1 illustrates an aircraft 3 having tail-mounted gas turbine engines 6. The engines 6 each drive a fore propeller 9F and an aft propeller 9A which rotate in opposite directions about an axis.
FIG. 2 illustrates in greater detail the engine-propeller system of FIG. 1. To the left is a gas turbine engine 15 such as the F404 type manufactured by the assignee of the present application. For purposes of the present invention, the gas turbine engine 15 can be considered as a gas generator which generates a high energy gas stream 33 and supplies the gas stream 33 to a propeller stage 36.
The propeller stage 36 extracts energy from the gas stream 33 directly, by means of low-speed counterrotating turbine blade sets. (This is in distinction to the common approach of using a high-speed turbine whose speed is reduced en route to a propeller by a reduction gearbox.) A first set of blades 39 extracts energy from the gas stream 33 and spins fore propeller 9F. A second set of blades 42 spins the aft propeller 9A, but opposite in direction to fore propeller 9F. Bearings 47 support the blade sets and propellers and allow this counterrotation.
A pitch-change mechanism 52 for changing the pitch of the propellers 9A and 9F is shown schematically. It is desirable to control the pitch-change mechanism 52 so that the propeller pitch is proper under the prevailing operating conditions of the aircraft.